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Re: [HACKERS] Re: Multi field hash indexes

From: Michael Meskes <meskes(at)topsystem(dot)de>
To: hannu(at)trust(dot)ee (Hannu Krosing)
Cc: pgsql-hackers(at)postgresql(dot)org (PostgreSQL Hacker)
Subject: Re: [HACKERS] Re: Multi field hash indexes
Date: 1998-03-17 13:00:01
Message-ID: 199803171300.OAA28075@gauss.topsystem.de (view raw or flat)
Thread:
Lists: pgsql-hackers
Hannu Krosing writes:
> It can be complicated (especially for extensible hashing) but the
> theory
> for this seems to be on in the database handbooks under the name of
> 'segmented hash' or some like fancy name.
> 
> The trick is to hash each field separately and then have a concatenation
> 
> of the hash values.

More or less.

> so assuming that for fields (a,b,c) values (120, 'friday', 3.1415927)
> hash
> to 'aa', 'bb', 'cc' the hash value of the whole tuple will be 'aabbcc'

I had a research project on this several years ago and we found that all
these multidimensional hash trees have one or the other problem. But one
index type named Balanced Multidimensional Hash Tree (from the top of my
head) performed excellently by combining hash and tree structures. 

Michael

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